The broad aims of our research are to understand the molecular mechanisms and physiological consequences of regulating eukaryotic gene expression by post-transcriptional processes. A large amount of important biological regulation occurs post-transcriptionally, but many of the mechanisms are not well understood. We will study selective mRNA degradation, the processes by which specific mRNAs are actively eliminated from cells. Regulated turnover of mRNAs profoundly influences gene expression, and our broad goals are to understand the mechanisms and consequences of that regulation. We focus on the accelerated degradation of mRNAs that contain premature translation termination codons. Messenger RNAs that contain premature stop codons are degraded more rapidly than their wild-type counterparts, a phenomenon termed "nonsense-mediated mRNA decay" (NMD). NMD occurs in all eukaryotes tested, and its action substantially impacts human genetic disease. Approximately one third of all sequenced point mutations that cause human genetic disease are nonsense or frameshift mutations, and most of these are known or predicted to elicit NMD of the mutant mRNA. We will study seven C. elegans genes required for NMD (smg-1 through smg-7) and will investigate their genetic and molecular properties. Our work falls into three broad categories: (1) investigating the molecular mechanisms of NMD;(2) investigating the relationship of NMD to RNA interference;and (3) investigating the biological function of NMD in wild-type animals. Our Specific Aims are: 1. We will investigate the structure of mRNPs containing premature translation termination codons. Such mRNPs differ from those not containing premature stop codons by selectively binding the SMG-2 protein. 2. We will study the roles of SMG-2 phosphorylation and dephosphorylation in NMD. 3. We will investigate a recently discovered relationship between NMD and RNAi. We will test whether NMD contributes to gene silencing via endogenous RNAi. 4. We will identify most or all of the endogenous mRNA targets of NMD. Our long-range goals are to understand both the molecular mechanisms of mRNA turnover and the biological roles of NMD. Our experiments contribute to these goals by identifying proteins that are required for NMD, by describing their biochemical activities in vivo, and by investigating the role of NMD during normal growth and development.